Variants of the normal mitochondrial DNA are described in Zea mays; these variants confer cytoplasmically inherited male sterility on the whole plant. One variant (T type) is also sensitive to toxin(s) produced by Helminthosporium maydis race T. We are studying the organization of the mitochondrial genome and comparing the restriction endonuclease digestion patterns of normal and variant mitochondrial DNA; we plan to clone the divergent sequences for a more thorough examination of the type of changes in the variant genomes. We are also studying the toxin(s) produced by H. maydis; four pathotoxic toxins have been crystallized and characterized thus far but none of them are host-specific. The host specific toxin has been greatly purified, however, and we plan to elucidate the chemical nature of the toxin this year.